Pore pressure of formation fluids in a subsurface formation can either show normal or hydrostatic behaviour whereby the pore pressure is hydrostatically determined by the weight of the overhead fluid column, or abnormal or non-hydrostatic behaviour whereby other factors than the weight of the overhead fluid column affect the pressure of the formation fluid. There are generally two types of non-hydrostatic behaviour identified, being over pressure whereby the pore pressure exceeds the hydrostatically determined pore pressure and under pressure whereby the pore pressure is less than the hydrostatically determined pore pressure.
Generally, the pore fluid pressure in a subsurface formation is determined with a so-called repeat formation test within the region of interest, or particularly within a depth range of interest.
In particular in the field of exploration drilling, there is a desire to predict an abnormality in pore fluid pressure in a region of interest that lies ahead of the drill bit. It is particularly desirable to be able to predict a region of over pressure in the formation, for an over pressure can give rise to influx of formation fluid in the bore hole, a so-called kick which can result in a blow-out.
Currently available pore fluid pressure prediction techniques in the art are normally based on determining a deviation in the porosity from a normal compaction trend of the formation, which determines the porosity. Such a technique is thus based on determining undercompaction and is referred to as an undercompaction technique. In such techniques it is generally assumed that over pressure is associated with abnormally high sediment porosity. However, over pressure does not always have a strong porosity-based signature, because over pressure can be caused by varying geological processes and are frequently related to complex geological structures, such as diapirs and overthrusts.